Traditionally, hemoglobin measurements have been carried out based on in-vitro analysis of subject's blood. Measurement devices known as co-oximeters determine hemoglobin concentration from a blood sample by measuring spectral light transmission/absorption through a hemolysed blood sample at several wavelengths typically between 500 and 650 nm. An example of a portable co-oximeter is presented in U.S. Pat. No. 7,029,628.
A major drawback related to co-oximeters is that the measurements are invasive, i.e. require a blood sample to be taken from the patient. Furthermore, the co-oximeters are rather expensive laboratory devices and require frequent service and maintenance.
Non-invasive optical hemoglobin measurements in-vivo are based on artificially induced changes in the blood flow of the patient.
U.S. Pat. Nos. 6,400,972, 6,711,424, and 6,587,704 disclose measurement devices based on a so-called occlusion-release (OR) measuring technique. A typical OR based measurement device utilizes a ring-shaped cuff applied to the patient's finger. The device is further provided with a pressurizing arrangement to produce a state of temporary blood flow cessation in the finger by applying an over-systolic pressure and a state of transitional blood flow by releasing the over-systolic pressure. Measurement sessions are carried out during various states of blood flow and the blood absorption characteristics during the said states are analyzed to determine the concentration of a blood constituent, such as hemoglobin.
U.S. Pat. Nos. 5,372,136, 6,671,528 B2, and 6,873,865 B2 disclose measurement devices in which the artificially induced changes in the blood flow are combined with light transmission/absorption measurements at two or more wavelengths. The wavelengths include an isobestic wavelength (805 nm) and a wavelength at which water absorption is high (1310 nm or 1550 nm) to detect the concentrations of hemoglobin and water, respectively.
Compared to invasive techniques, non-invasive optical hemoglobin or hematocrit measurements have clear advantages, which include the elimination of both painful blood sampling and the risk of infection. Furthermore, non-invasive measurements are simpler to carry out and require less training of the nursing staff.
However, there are also several drawbacks related to the above non-invasive techniques. First, the devices are rather complicated since the optical measurement involves synchronized operation of the optical and pneumatic components of the measurement device. Second, the measurement cannot be carried continuously, but requires a certain measurement period for each measurement. Typically, the measurement cycle is manually initiated, which makes the devices suitable for spot checks after the need for the hemoglobin measurement has been recognized based on subject's symptoms. Consequently, the current non-invasive hemoglobin meters cannot be used for alarming of a sudden hemoglobin loss. Third, normal low-cost silicon detectors, which are used in standard pulse oximeters, can be used only in the near infrared region, since their response ends at a wavelength of about 1000 nm. Therefore, more expensive detector technology must be used for enabling measurement of water absorption in the short-wavelength infrared region, such as at wavelengths around 1300 nm.
The present invention seeks to eliminate the above drawbacks and to bring about a novel mechanism for non-invasive and continuous determination of the concentration of a blood substance, such as hemoglobin.